CHAPTER VIII 



THE ENVIRONMENT OF MENTAL 

 EVOLUTION 



EVEN yet we have not seen the full importance of air, that 

 is, of climate, upon man's evolution. We have seen how 

 it influenced his body, but why should his mind be even 

 more sensitive than his body? Why should the mental optimum 

 be lower than the physical? Why should variable weather and 

 cold waves stimulate business and invention as much or more than 

 they stimulate health? The answer seems to be found in the 

 climatic conditions under which man's mind made its most rapid 

 evolution. We have seen that the greatest crises in the evolution 

 of the human body were associated with epochs of severe climate. 

 Other factors have indeed played a part, but as Lull puts it: 

 "Changing environmental conditions stimulate the sluggish evolu- 

 tionary stream to quickened movement. Whenever it has been 

 possible to connect cause and effect, the immediate influence is 

 found to be generally one of climate." 



This is preeminently true of mental evolution. Throughout the 

 Glacial Period the form of the lands was essentially the same as 

 now. The one thing that was profoundly different was the climate. 

 It must have worked in two ways. First, it must have weeded out 

 those members of the human race who could not endure its rigors, 

 or who did not migrate to milder climates. Second, as we shall 

 see in the next chapter, it probably was the actual cause of what 

 the biologist calls mutations, so that new human types arose. 



In order to appreciate the part played by climate in the evolu- 



